1. Field of the Invention
This invention relates to steam turbine power plants, and in particular, to an arrangement for operation of a pressurized water reactor power plant during extended fuel cycle operation.
2. Description of the Prior Art
A typical nuclear steam power plant comprises an interconnected arrangement of two loops; one loop comprising a primary, or nuclear, side, with the other comprising a secondary, or steam, side. The primary side includes a nuclear reactor element and a steam generator element, and a closed conduit arrangement carrying therein a pressurized coolant which passes through both the reactor and the steam generator element. The steam side comprises a series-connected arrangement including the steam generator element, a high-pressure and a low-pressure turbine, a condenser, and a bank of feedwater heaters.
In operation, the pressurized fluid coolant within the conduits of the primary loop takes the heat produced by reactions within the reactor core and transfers that heat within the steam generator element to boiler feedwater, which is, in turn, raised in temperature and converted to steam. The steam exits the steam generator element and is permitted to expand through both the high- and low-pressure turbines. The expanding steam acts against rotating elements within the turbines and converts the pressure energy of the steam to rotational mechanical energy. After expansion, the steam is condensed within the condenser and conducted back to the steam generator element. Each of the plurality of feedwater heaters raise the temperature of the feedwater prior to its reintroduction into the steam generator element.
In general, it is known that for the steam side, the work produced thereby is equal to the product of the amount of heat added multiplied by a factor known as conversion efficiency. Symbolically, this is indicated by the relation EQU W = (Q.sub.a)(N)
it is also known that the amount of heat added, Q.sub.a, is equal to the mass flow rate, G, multiplied by the energy added per pound of fluid, .DELTA.h. Further, it is known that N, the conversion efficiency, is a function of the final feed temperature.
After the reactor has been operating for a considerable period of time and the control rods therein have been extracted to their fullest extent, the throttle pressure of the steam entering the steam side begins to decrease. Thus, for a given size orifice within the high-pressure turbine, the flow therethrough decreases due to the decrease in the throttle pressure. As the throttle pressure decreases, G (the mass flow) decreases, and, therefore, the amount of heat added, Q.sub.a, decreases, leading to a concomitant decrease in the work output.
The prior art, especially that shown in the copending Buscemi, Nusbaum and Silvestri application, Ser. No. 419,746, filed Nov. 28, 1973 and assigned to the assignee of this invention, attempts to increase the reactivity levels within the reactor at "stretch-out" (that is, after the full extraction of all control rods) through several schemes. One such scheme is to increase the mass flow by completely bypassing the high-pressure element. An alternative scheme is to lower the temperature level of the final feed by either individually or collectively closing the extractions from the turbine elements to the feedwater heaters, or, alternatively, shunting any or all of the individual heaters. In this way, since .DELTA.h, dependent upon final feed temperature, increases, Q.sub.a also increases.
However, from the work output standpoint, the expedient of the above-mentioned application which increases mass flow has the effect of decreasing work output because it bypasses the high-pressure turbine. Also, the expedients which shunted feedwater heaters resulted in a decrease in conversion efficiency since the temperature of the final feed decrease. Applicant in his invention described herein is able to accommodate both an increase in the amount of heat added to the system (Q.sub.a) and also contemporaneously maintain a high conversion efficiency (N).